In the photostimulable phosphor imaging system, as described in U.S. Pat. No. Re. 31,847 reissued Mar. 12, 1985 to Luckey, a photostimulable phosphor sheet is exposed to an imagewise pattern of high energy short wavelength radiation, such as x-radiation, to record a latent image pattern in the photostimulable phosphor sheet. The latent image is read out by stimulating the phosphor with a relatively long wavelength stimulating radiation, such as red or infrared light. Upon stimulation, the photostimulable phosphor releases emitted radiation of an intermediate wavelength, such as blue or violet light, in proportion to the quantity of short wavelength radiation that was received. To produce a signal useful in electronic image processing, the photostimulable phosphor sheet is scanned in a raster pattern by a beam of light produced for example by a laser deflected by an oscillating or rotating scanning mirror, and the emitted radiation is sensed by a photodetector such as a photomultiplier tube, to produce the electronic image signal.
In the conventional photostimulable phosphor imaging systems, the photostimulable phosphor sheet is turbid to both stimulating and emitted wavelengths of light. In such a turbid phosphor system, the minimum pixel size, and hence the resolution that can be achieved, corresponds to the scattered area of the scanning beam within the stimulable phosphor sheet itself. It has been proposed that the resolution of a photostimulable phosphor imaging system may be greatly improved by making the photostimulable phosphor sheet transparent, thereby enabling a reduction in the effective pixel size, since the apparent size of the beam is not increased by scattering. See for example, the article entitled "Laser-Stimulable Transparent CsI:Na Film for a High Quality X-ray Image Sensor" by Kano et al, Applied Physics Letters 48(17), Apr. 28, 1986. Since the MTF (Modulation Transfer Function--a measure of the ability of the system to record details) of the transparent photostimulable phosphor imaging system is limited mainly by the effective size of the scanning beam of stimulating radiation, the x-ray absorption of the sheet may be increased by making it thicker, without increasing the effective size of the scanning beam. In this way, the signal-to-noise ratio of the x-ray detector may be improved. In the conventional turbid phosphor sheets, the thickness was limited by the spreading of the scanning beam in the turbid phosphor. Unfortunately, the transparent photostimulable phosphor sheet has a drawback, in that a large fraction of the emitted light is totally internally reflected within the phosphor sheet and is not collected by conventional light detectors. The only emitted light that escapes from the surface of the photostimulable phosphor sheet is that which is emitted in the solid angle subtended by the light rays incident at less than a critical angle to the surface. For a transparent photostimulable phosphor sheet having an index of refraction of 1.6, this means that only about 11% of the light escapes from the top of the sheet, and another 11% from the bottom, the remainder of the light is trapped within the sheet by total internal reflection.
One solution to this problem is to employ a photostimulable phosphor sheet that comprises a photostimulable phosphor dispersed in a polymeric binder. The polymeric binder is selected such that its index of refraction matches that of the phosphor at the stimulating wavelength, but does not match that of the phosphor at the emitted wavelength. Thus, the emitted wavelength is scattered and is not trapped by total internal reflection, while the benefits of the transparent phosphor sheet are achieved for the stimulating wavelength. See Canadian Pat. No. 1,175,647 issued Oct. 9, 1984 to DeBoer & Luckey. Although this solution is ideal for a phosphor-binder type photostimulable medium, it does not solve the problem for an isotropic photostimulable medium such as the fused phosphor described in the above-referenced Kano et al article. Furthermore, even in a phosphor-binder type photostimulable medium, the desired indices of refraction are difficult to achieve in practice. A rapid change in the index of refraction of a material with changes in wavelength, which is necessary for the phosphor-binder system to be transparent to stimulating wavelength and turbid for the emitted wavelength of light, is generally associated with an absorption peak. Obviously, the presence of an absorption peak near the wavelengths of interest is to be avoided if maximum efficiency is to be achieved from the system.
It is therefore the object of the present invention to provide a light detector for use with a transparent photostimulable phosphor image recording medium having improved light collection efficiency.